The first thing which was introduced as a regulator was a fly-wheel. There was a fly-wheel of a certain weight, and the force which was applied to the clock had to turn the wheel against the resistance of the air; but that did not answer well, and the first tolerable arrangement was suggested by Henry de Wyck, who constructed a bell and a clock in 1364, in which the fall of the weight was prevented by an oscillating balance, similar to that shown in Fig. 85. [Illustration: FIG. 86.—The Crown Wheel. ] Here we see what is called the crown wheel (S S, shown in plan, Fig. 86), on which the escapement depends, and into the teeth of which work two pallets, P_{1} P_{2}, which are placed on a vertical axis pivoted above and below. Now if we suppose a weight attached to the cord passing over a drum, so as to propel the intermediate wheels and pull them round, the crown wheel tends to rotate, but is prevented from moving until the pallets give way. Let us see how the clock goes. When the bottom tooth, presses against the pallet P_{1}, in order to make it get out of the way and enable the wheel to go on, it twists the rod and moves the horizontal bar M M, on which are several saw-like teeth, on the intervals of which, as in the modern steelyard, weights are placed, so that the wheel pushes away the pallet and makes the horizontal beam describe a part of a circle. And what happens is this:—the upper pallet is turned out of its position and driven into the upper teeth of the wheel, and driven out by the further revolution of the wheel, so that the fall of the weight depends on the oscillations of the horizontal beam which carries the weights. The clock was regulated by the distance of the weights from the pivots on which the balance swung. Such was the form of clock used by Tycho Brahe, but with little success, for it was extremely irregular in its action, and Tycho still had to compare the position of one star with another instead of trusting to his clock. There is no necessity to say much regarding the train of wheels between the weight or spring and the escapement. Their office is simply to create a great difference in velocity of rotation between the wheel turned by the weight or spring and the escape wheel, so that a slow motion with great force may be transformed into a quick motion with small force. The train of wheels is so arranged, by the consideration of the number of teeth in the wheels, that one wheel shall go round once an hour, and another once a minute, so that the first may carry the minute-hand and the other the second-hand. The hour-hand wheel is also geared to the minute-wheel, so that it shall turn once in twelve hours or twenty-four hours, according to the purposes for which the clock is required. Weights are usually used when space is no object, being more regular in their action than springs; but the latter are used for chronometers and watches, and other portable time-keepers.